Composite clay materials for removal of sox from gas streams

ABSTRACT

A method for preparing and using compositions including a smectite clay and a base or base precursor which reacts with SO x  in a hot flue gas is described. The base or base precursor is preferably the dispersed phase in the bulk phase of the clay. The compositions are heated to form the base which reacts with SO x  in the flue gas.

This is a divisional of copending application Ser. No. 07/553,254 filedon Jul. 16, 1990.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to alkali and alkali earth oxides or carbonatessupported on clays where the host clay belongs to the smectite group,and to a method of preparing them. More particularly, it relates to themethod of developing improved composite materials for use in SO_(x),(sulfur dioxide and sulfur trioxide), removal from flue gas.

(2) Prior Art

The first example of flue gas scrubbing for sulfur dioxide controloccurred in London, England, in 1933. However, the application of thistechnology to coal-fired utility boilers in the United States did notbegin until the 1970's. The first large-scale application of flue gasscrubbing using lime or limestone was installed in 1964, in the SovietUnion. This facility was followed by an installation at a large sulfuricacid plant in Japan in 1966. In 1970, the Clean Air Act Amendments wereadopted. This legislation provided for enforcement, by the U.S.Environmental Protection Agency (EPA), of SO_(x) emissions limits forpower plants constructed or modified after Aug. 17, 1971. This Actspurred extensive flue gas desulfurization (FGD) research. As of January1984, calcium-based, wet, throwaway systems (including lime, limestone,and alkaline-ash systems) accounted for 84 percent of existing andplanned FGD capacity. The Clean Air Act was amended in 1977 to requirefurther control of SO_(x) emissions. Increasing federal regulations andthe high cost to construct and operate existing wet FGD units haveencouraged continued research on new or modified flue gas cleanupprocesses.

Controlling the emissions of SO_(x) from power plants is a world-wideproblem due to its relationship to "acid rain". Therefore, research intoits control is a global effort Example of a recent patent using calciumbased systems to reduce SO_(x) emissions is Thompson and Nuzio, U.S.Pat. No. 4,731,233. In most cases a commercial source of limestone orlime is used due to cost effectiveness and available quantities.

There are a number of ways to control SO_(x) emissions in existing powerplants or features that can be included in construction of new powerplants These approaches can be classified according to the position inthe combustion system at which pollutant control technology is applied.Precombustion control involves removal of sulfur, nitrogen and ashcompounds from the fuel before it is burned. In most cases this involvesapplication of coal-cleaning technology. Combustion control includesstaged combustion, boiler limestone injection, and fluidized-bedcombustion with limestone addition. Post-combustion control involvesremoval of pollutants after they have been formed but before they arereleased into the atmosphere. This would include in-duct dry sorbentinjection, induct spray drying and combined electrostatic precipitator(ESP)/fabric filter sorbent injection (Jozewicz, W., Chang, J. C. S.,Sedman, C. B. and Brna, T. G., React. Solids, 6 243 (1988)).

Flue gas treatment systems can be classified as either wet or dry basedon the moisture content of the treated flue gas and the spent sorbent.Wet systems completely saturate the flue gas with water vapor. The fluegas is contacted with a liquid or slurry stream. Dry systems contact theflue gas with a dry or wet sorbent but never include enough water forcomplete saturation of the flue gas. Dry systems result in a dry productor spent sorbent material, while wet systems result in either a slurryor a sludge.

Although calcium based systems are the major source of SO_(x) controlthey are not without problems Agglomeration of particles can be aserious problem that results in a less than optimal conversion toCaSO_(x), (CaSO₃ and CaSO₄) The activity of the calcium speciesdecreases as its particle size increases Also CaSO_(x) occupies morevolume than CaO, the common active species Therefore, an increase involume occurs as the reaction proceeds, which causes a loss in theoriginal porous character of the CaO. This results in a blockage ofSO_(x) and 02 to the active CaO centers (Gullett, B. K. and Blom, J. A.,React. Solids, 3 337 (1987): Gullett, B. K., Blom, J. A. and Cunningham,R. T., React Solids, 6 263 (1988); Chang, E. Y. and Thodes, G., AIChEJ., 30 450 (1984); Thibault, J. D., Steward, F. R. and Ruthven, D. M.,Can. J. Chem. Eng., 60 796 (1982)).

Prior art in this field has used limestone, lime or hydrated lime as aprecursor for the active CaO species or has used Ca(OH)₂ as the activespecies Generally, the active species has been used as a bulk phase andnot as a dispersed species (Chang, J. C. S. and Kaplan, N., Envir.Prog., 3 267 (1984); Gullett, B. K., Blom, J. A. and Cunningham, R. T.,React. Solids, 6 263 (1988); Chang, E. Y. and Thodes, G., AIChE J., 30450 (1984); Fuller, E. L. and Yoos, T. R., Langmuir, 3 753 (1987)).Recent work has concentrated on the addition of fly ash to Ca(OH)2 toenhance its activity in SO_(x) control (Jozewicz, W. and Rochelle, G.T., Envir. Prog. 5 219 (1986); Jozewicz, W., Chang, J. C. S., Sedman, C.B. and Brna, T. G., JAPCA, 38 1027 (1988); Jozewicz, W., Jorgensen, C.,Chang, J. C. S., Sedman, C. B. and Brna, T. G., JAPCA, 38 796 (1988);Jozewicz, W., Chang, J. C. S., Sedman, C. B. and Brna, T. G., React.Solids, 6 243 (1988); Jozewicz, W., Chang, J. C. S., Sedman, C. B. andBrna, T. G., EPA/600/D-87/095, (NTIS PB87-175857/AS); Jozewicz, W.,Chang, J. C. S., Sedman, C. B. and Brna, T. G., EPA/600/D-87/135, (NTISPB87-182663).

The fly ash is a siliceous material and the formation of various calciumsilicates can occur Several diatomaceous earths, montmorillonite clays,and kaolins have also been identified as containing reactive silica(Jozewicz, W., Chang, J. C. S., Sedman, C. B. and Brna, T. G., React.Solids, 6 243 (1988)).

It is therefore an object of the present invention to provide a methodfor preparing improved sorbent compositions including a smectite clay asthe bulk phase and with a relatively smaller amount of a basic compoundas the active phase Further, it is an object of the present invention toprovide improved compositions which are more active over time, thus moreeffective in SO_(x) removal. Further still, it is an object of thepresent invention to provide compositions which are relativelyeconomical to prepare and use. These and other objects will becomeincreasingly apparent from the following description and the drawings.

IN THE DRAWINGS

FIG. 1 is a graph showing a Cu K alpha XRPD pattern of the A:Z, A=1,Z=1, CaCO₃ :Na-montmorillonite composite designated Sample 1.

FIG. 2 is a graph showing a TGA profile of Sample 1.

FIG. 3 is a graph showing the initial SO₂ uptake of Samples 1, 3 andprecipitated CaCO₃.

FIG. 4 is a graph showing a Cu K alpha XRPD pattern for Sample 17,1.5:1, Ca(CH₃ CO₂)₂ :Na-montmorillonite.

FIG. 5 is a graph showing the temperature dependence of SO₂ uptake forthe 1:1 ratio of CaCO₃ :Na-Montmorillonite composite, for Samples 1, 22,23 and 24.

GENERAL DESCRIPTION

The present invention relates to a method for preparing a compositematerial useful for removing SO_(x) from a gas stream which comprises:providing a suspension of a smectite clay in water; providing a basiccompound selected from the group consisting of alkali metal and alkalineearth metal salts and bases in the suspension with the clay; and dryingthe suspension to provide the composite material, wherein when thecomposite material is heated, preferably to at least about 500° C., theSO_(x) is removed from the gas by the basic compound.

Further, the present invention relates to a method of preparing acomposite material, capable of removing SO_(x) from a gas streamcomprising the steps: providing a suspension of a smectite clay inwater; introducing a quantity of a basic compound selected from thegroup consisting of a base and a base precursor into the claysuspension; and drying the suspension in air, wherein when the compositematerial is heated, preferably to at least about 500° C., the SO_(x) isremoved from the gas.

Further still, the present invention relates to a method for preparing acomposite material capable of removing SO_(x) from a gas streamcomprising the steps of: providing a suspension containing a smectiteclay in water; dissolving an amount of sodium carbonate in thesuspension of the clay; adding a soluble alkaline earth metal salt instoichiometric amount for reaction with the sodium carbonate to form analkaline earth metal carbonate precipitate in the suspension with theclay; washing the composite; and drying the suspension to provide thecomposite material, wherein when the composite material is heated,preferably to at least about 500° C.

Finally the present invention relates to a composition for use inremoving SO_(x) from a gas stream when the composite material is heated,preferably to at least about 500° C., which comprises in admixture: abasic compound selected from the group consisting of alkali metal andalkaline earth metal salts and bases; and a smectite clay, wherein thebasic compound is provided in an aqueous suspension with the clay andthen dried to form the composition.

The present invention provides a method for production of compositematerials consisting of alkali or alkali earth metal/smectite claycomposites, of varying alkali or alkali earth metal/clay ratios than hasheretofore been known in the prior art, specifically by causing theprecipitation of CaCO₃ or Ca(OH)₂ from salts onto and into the clayparticle while the clay is in suspension and by the impregnation ofsodium or calcium species into the clay particle by the addition of anaqueous solution of alkali or alkali earth species to a suspension ofclay to produce these ratios. The resulting materials are used to removeSO_(x) from flue gas.

In accordance with one method of the invention, a 0.5 to 1.5 weightpercent aqueous suspension of clay was initially prepared An aqueoussolution of Na₂ CO₃ was added dropwise to the clay suspension while itwas stirred

This was followed by a similar addition of CaCl₂.2H₂ O. The addition ofthe calcium species caused the precipitation of CaCO₃. The amount of Na₂CO₃ and CaCl₂.2H₂ O can be varied to provide the desired weight ratio ofCaCO₃ to clay. The product was washed with deionized distilled water,either by centrifugation/decantation or by dialysis, to remove theexcess chloride and sodium ions before drying at room temperature or inan oven at 100° C. The wash was checked with a silver nitrate solutionfor precipitation of silver chloride. A negative test for silverchloride indicated that the chloride had been removed. Washing thepreparation was preferred, because reactivity with SO_(x) was diminishedif no attempt was made to remove the chloride The adverse effect ofchloride on SO_(x) removal has also been verified by another study whichevaluated the effects of magnesium and chloride ions on the performanceof limestone-regenerated dual alkali processes under closed-loopoperating conditions (Chang, J. C. S., Kaplan, N. and Brna. T. G. in"Fossil Fuels Utilization: Environmental Concerns" (Eds. R.Markuszewski, B. Blaustein) Chap. 15). Limestone reactivity decreasedwith the increase of chloride ion concentration. The effect wasespecially pronounced after a concentration of 80,000 ppm was reached.

X-ray powder diffraction (XRPD) indicates that the clay retains itsoriginal layered structure with a basal spacing of approximately every10 Å. Crystalline CaCO₃ is also present in the XRPD pattern.

The precipitation of Ca(OH)₂ was carried out using a similar method. A50 wt.% solution of NaOH containing the desired amount of OH⁻ was addedto water. The solution was added dropwise to the clay suspension whilestirring. An aqueous solution of CaCl₂.2H₂ O was added in a similarmanner after addition of the NaOH solution.

In situ precipitation resulted in smaller CaCO₃ particle size than canbe attained through a physical mixture of CaCO₃ and the clay.

An oxidation catalyst was also added to some of the compositepreparations. Iron was added in the form of FeCl₃.6H₂ O or Fe(NO₃)₃.9H₂O. Fe₂ O₃ was formed upon thermal decomposition of the salts; andcatalyzed the oxidation of SO₂ to SO₃. SO₃ reacts with calcium oxide atlower temperatures than SO₂ resulting in a wider temperature window forthe removal of SO_(x) from the gas stream. Various transition metals canbe used as oxidation catalysts.

Prior to exposure to SO₂ the product was heated to 900° C. in air.Thermal gravimetric analysis (TGA) indicated the loss of H₂ O and CO₂based upon mass measurements of the sample. The thermal treatment causedthe decomposition of CaCO₃ to CaO. It also caused a severe drop in theclay intensity present in the pattern with only a low intensityremaining at 9.4 Å for the CaCO₃ /Na-montmorillonite composite.

A second method of preparing a highly dispersed alkali or alkali earthmetal species was the impregnation of a water soluble sodium or calciumcompound onto the clay. As previously described, the desired amount ofdissolved alkali metal or alkali earth metal carbonate species was addedto the clay suspension while the suspension was stirred. Calciumformate, calcium acetate and sodium bicarbonate have been used asexample preparations, but other soluble alkali metal and alkaline earthmetal salts may be used. Washing was not necessary in these latterpreparations that were chloride free. The suspension was air dried on aglass plate at room temperature.

An XRPD pattern of calcium formate and Na-montmorillonite in a 1:1weight ratio contained crystalline calcium formate peaks and d.sub.(001)spacing typical of the clay (10.9 Å). A small peak also occurred at ad-spacing of 22.7 Å, indicating that some intercalation of the formatespecies occurred In contrast, the XRPD pattern for a calciumacetate/Na-montmorillonite composite prepared using the impregnationmethod did not exhibit crystalline calcium acetate. Instead, a majorreflection occurred at a d-spacing of 24.0 Å, indicating that most of,the calcium acetate species formed a nanocomposite with the clay byintercalation into the interlayer of the clay host forming a newcomposition of matter. (T. J. Pinnavaia in "Chemical Physics ofIntercalation", Ed. by A. P. Legrand and S. Flandrois, NATO ASI Series,Series B: Physics Vol. 172, pp 233-252, (1987)).

The products resulting from this invention constitute a calcium/claycomposite in the case of precipitated CaCO₃ from the previouslymentioned salts or impregnation of Ca(CHOO)₂, Ca(CH₃ CO₂)₂ or NaHCO₃. Asodium/clay composite is formed in the case of impregnated NaHCO₃. Amore intermediate interaction between the clay and the calcium acetateoccurred during the impregnation method, as evidenced by a change in theclay XRPD pattern, resulting in a regularly intercalated claynon-composite.

Although CaCO₃, NaHCO₃, Ca(CHO₂)₂, and Ca(CH₃ CO₂)₂ are suitable basesor base precursors for SO_(x) removal in the temperature range 100°-900°C. by composite formation with smectite clay, certain calcium salts suchas calcium hydroxide (quicklime), Ca(OH)₂, is not a suitable reagent forthis purpose because the salt is too unstable on the clay support athigh temperatures, presumably due to chemical reaction with the claysupport, and its ability to remove SO_(x) from a gas stream is lost.

The particle size of the smectite clay is preferably less than about 2microns. A particle size between about 0.1 and 2 microns is preferred.

A weight ratio of about 1 to 3 and 5 to 1 for salt to clay is preferred.Any weight ratio which produces effective SO_(x) removal from flue gascan be used, preferably where the bulk phase is the clay.

The composite material can be dried by various means including spraydrying, tray drying and the like. The solids can be removed fromsolution by centrifuging with intermediate washing steps with water.

SPECIFIC DESCRIPTION Example 1

Na-montmorillonite from Crook County, Wyoming USA was selected as therepresentative member of the smectite family of 2:1 layer latticesilicates. A 1.4 weight percent, wt.%, of clay was dispersed indeionized distilled water. An upper limit of 2 micron particle size wasachieved by sedimentation in water and application of Stokes' law ofsettling under gravity. The procedure was followed twice. Sedimentationalso removed quartz and other insoluble impurities that may have beenpresent in the clay. After purification the clay was air dried on aglass plate or stored in an aqueous suspension.

A desired weight ratio, A:Z, of CaCO₃ :Na-montmorillonite was achievedby preparing a 1.4 wt.% solution of Na-montmorillonite in deionizeddistilled water. A total weight of 20 grams of solution was used. In theexample of A=Z=1, 0.28 grams of CaCO₃ or 2.8 x 10⁻³ mole was desired.Therefore, 2.8×10⁻³ mole of Na₂ CO₃ was dissolved in deionized distilledwater with a total solution weight of 20 grams. The Na₂ CO₃ solution wasslowly added to the Na-montmorillonite suspension while stirring. Asimilar solution of CaCl₂.H₂ O was added to the solution after additionof the Na₂ CO₃ solution was complete. The sodium salt is added first todiscourage cation exchange of the calcium with the sodium in themontmorillonite.

After preparation the product was washed and centrifuged repeatedly withdeionized distilled water to remove excess chloride and sodium ions. Thewash was tested with a solution of silver nitrate to determine theabsence of chloride ions. The product was dried on a glass plate atambient temperature. An XRPD pattern of the product is shown in FIG. 1.The reflection at 5.8° (15.1 Å) is the major Na-montmorillonite peak andthe peak at 29.5° (3.0 Å) is the major peak of CaCO₃.

A sample prepared as described above was evaluated and was shown to beactive for SO₂ removal from a gas mixture. The sample was heated to 900°C. at a rate of 5° C./min and maintained at 900° C. for 30 minutes priorto the introduction of 5000 ppm SO₂ for 1 hour in flowing air. Theuptake of SO₂ is illustrated in FIG. 2. The sample removed an 88% CaCO₃conversion, with 19% conversion occurring within the first minute. Theconversion is based upon the following reaction: ##STR1##

Examples 2 and 3

Examples 2 and 3 provided products, designated samples 2 and 3,respectively, that were prepared utilizing the procedures of Example 1,with the weights of Na₂ CO₃ and CaCl₂.2H₂ O varied to yield respectivevalues of A=3, Z=1; A=0.33, Z=1. Samples of 2 and 3 were tested for SO₂uptake following the procedure cited in Example 1. Sample 2 exhibited atotal CaCO₃ conversion of 100% with 14% of the conversion occurringduring the first minute of the reaction Sample 3 gave a total CaCO₃conversion of 100%, with conversion after 1 minute of 36%. FIG. 3illustrates the differences in initial SO₂ uptake by Samples 1, 3 andCaCO₃ caused by varying the CaCO₃ :Na-montmorillonite weight ratio.

Examples 4 to 6

A series of 1:1 CaCO₃ :smectite clay composites in accordance with themethod of Example 1, were prepared using other members of the smectitefamily in place of purified montmorillonite, Bentone EW P&G GST-865(Sample 4); nontronite (Sample 5); fluorohectorite (Sample 6). Thesecomposite preparations were tested for SO₂ reactivity according to themethod described in Example 1. After 1 hour of reaction, the Bentonepreparation gave a total conversion of 98%, the nontronite sample gave atotal conversion of 78% and the fluorohectorite sample gave a totalconversion of 90%.

Examples 7 to 9

A series of A:Z Na-montmorillonite samples in accordance with theinvention were prepared utilizing the procedures of Example 1, but thewashing procedure was eliminated and the drying temperature was varied.Samples 7 and 8 had A=1, Z=1 and A=0.33, Z=1, respectively, and weredried at room temperature. Sample 9 had A=1, Z=1 and was dried at 100°C. in air. Samples 7 to 9 had total conversions of 52, 59 and 42%,respectively, as determined by the test conditions cited in Example 1.The chloride ion interferes with the conversion.

Example 10

An iron-containing CaCO₃ :Na-montmorillonite composite, designatedSample 10, with A=Z=1 was prepared by a co-precipitation method usingFeCl₃.6H₂ O as the iron source. The addition of Na₂ CO₃ was carried outin accordance with the method described in Example 1, except thatFeCl₃.6H₂ O was added to the CaCl₂.2H₂ O solution to yield a compositecontaining an Fe content of 1.5 wt.%.

The sample exhibited a total CaCO₃ conversion of 42.5% after 1 hourreaction with SO₂ under the conditions described in Example 1.

Example 11

An iron containing CaCO₃ :Na-montmorillonite sample, designated Sample11, with A=Z=1 was prepared using Fe(NO₃)₃.9H₂ O as the source. A CaCO₃:Na-montmorillonite composite with A=1, Z=1 was prepared according tothe procedure described in Example 1. A 0.5 wt.% quantity ofFe(NO₃)₃.9H₂ O was dissolved in deionized distilled water and was addedto the CaCO₃ /Na-montmorillonite suspension. The product was air driedon a glass plate at room temperature. The sample showed a total CaCO₃conversion of 88% after 1 hour reaction with SO₂ under the conditionsdescribed in Example 1.

Examples 12 and 13

A 1:1 by weight composite of Ca(OH)₂ and Na-montmorillonite, designatedSample 12, was prepared by using NaOH, CaCl₂.2H₂ O andNa-montmorillonite as starting materials. An amount of a 50 weightpercent NaOH solution appropriate for the stoichiometric reaction withCaCl₂.2H₂ O was added to a stirred 50 gram of a 0.5 wt.% suspension ofNa-montmorillonite. After the addition of the NaOH, to the clay, anequivalent amount of a solution of CaCl₂.2H₂ O was added to the solutionto produce the 1:1 Ca(OH)₂ :Na-montmorillonite composite. The productwas washed with deionized distilled water as described in Example 1. Asecond sample, designated Sample 13, was prepared to afford a 3:1Ca(OH)₂ :Na-montmorillonite (Sample 13). The total conversions after 1hour reaction with SO₂ at 900° C. using the test conditions cited inExample 1 were 5 and 1%, respectively, for Samples 12 and 13. In theseexamples, Ca(OH)2 loses its ability to react with SO_(x) under thesereaction conditions when supported on smectite clay. Presumably theCa(OH)₂ reacts with the clay, forming a calcium silicate that is inertto SO_(x).

Examples 14 and 15

An amount of calcium formate, Ca(HCOO)₂, was dissolved in a minimumamount of deionized distilled water. The solution was slowly added withconstant stirring to a 1.4 wt.% slurry of Na-montmorillonite. The amountof Ca(HCOO)₂ added to the clay slurry was varied to yield composite witha Ca(HCOO)₂ :Na-montmorillonite ratio of 1:1, designated Sample 14, and3:1 designated Sample 15. The products were air dried at roomtemperature on a glass plate. Sample 14 gave a total conversion based onCa(HCOO)₂ of 60% after 1 hour reaction time with SO₂ and a conversion of24% after the first minute under the conditions described in Example 1.The conversions for Sample 15 were 94% and 14% after 1 hour and 1 minutereaction time, respectively.

Examples 16 to 19

A series of calcium acetate: Na-montmorillonite composite samples inaccordance with the invention, were prepared utilizing the procedures ofExamples 14 and 15 for the preparation of clay calcium formatecomposite. The amount of Ca(H₃ CCOO)₂.XH₂ O used was varied to yieldcomposite with Ca(CH₃ CO₂)₂ :Na-montmorillonite weight ratio of 1:1,designated Sample 16; 1.5:1 designated Sample 17; 0.33:1, designatedSample 18; 5.0:1, designated Sample 19.

An XRPD pattern of the product from Sample 17 is shown in FIG. 4. Thepattern does not exhibit any crystalline Ca(H₃ CCOO)₂ species but itdoes exhibit an increase in d-spacing for the clay structure, indicatingintercalation of the acetate species. The conversions for Samples 16 to19 after 1 hour reaction time with SO₂ were 52, 36, 15 and 78%, underthe conditions cited in Example 1.

Example 20

A sample of NaHCO₃ supported on Na-montmorillonite was prepared bydissolving the bicarbonate salt in a minimum amount of deionizeddistilled water and adding the solution slowly to a clay suspension withconstant stirring, and allowing the mixture to air dry on a glass plate.The NaHCO₃ :clay ratio was 1:2. The sample designated Sample 20, gave atotal conversion based on NaHCO₃ of 31% after 1 hour reaction with SO₂under the test conditions cited in Example 1.

Example 21

A quantity of sample 20 from Example 20 was tested for SO₂ reactivity at500° C. The sample was heated to 500° C. at a ramp rate of 5° C./minuntil a temperature of 500° C. was reached. The sample was maintained at500° C. for 30 minutes. An air stream containing 5000 ppm of SO₂ thenwas allowed to flow over the sample for 1 hour, total flow rate of 200cc/min while the sample was at 500° C. The sample was maintained at 500°C. for an additional 30 minutes without weight loss. The totalconversion after 1 hour reaction with SO₂ based on NaHCO₃ content was46%.

Example 22

A quantity of Sample 1 was tested for SO₂ uptake at 500° C. under theconditions cited in Example 21. The percent conversion for the samplewas 0%. The temperature is too low for effective conversion of theCaCO₃.

Example 23

A quantity of sample 1 was tested for SO₂ uptake at 700° C. by rampingthe temperature at 5° C./min until a temperature of 700° C. wasobtained. A temperature of 700° C was maintained for 30 minutes prior tothe introduction of SO₂ in air at a concentration of 5000 ppm. Thesample was maintained at 700° C. for an additional 30 minutes after theSO₂ without weight loss. The percent conversion based on CaCO₃ was 11%which indicated that the higher temperature was better.

Example 24

A quantity of sample 1 was tested for SO₂ uptake at 800° C. by rampingthe temperature at 5° C./min until a temperature of 800° C. wasobtained. A temperature of 800° C was maintained for 30 minutes prior tothe introduction of SO₂ in air at a concentration of 500 ppm. The samplewas maintained without weight loss at 800° C. for an additional 30minutes after the SO₂ had stopped. The percent conversion based on CaCO₃was 75%. The temperature dependence of SO₂ uptake for CaCO₃/Na-montmorillonite, 1:1 wt.% for samples from Examples 1, 22, 23 and 24is shown in FIG. 5.

Examples 25 and 26

Two of the preparations were tested for SO₂ reactivity under theconditions cited in Example 1 with additional moisture present in thegas stream. A quantity of sample 1 was run at a partial pressure of 0.22H₂ O present in the gas stream and yielded a total conversion of 111%,part of which may be due to moisture uptake by the sample. A secondsample from Sample 18 was tested at a partial pressure of 0.064 H₂ O andyielded a total conversion of 64%.

Examples 27 to 29

Samples 1, 10 and 11 were tested for SO₂ reactivity at 700° C. Thesamples were ramped to 900° C. as cited in Example 1 and then ramped to700° C. at 5° C./min. The sample was maintained at 700° C. for 30minutes before 5000 ppm of SO₂ was admitted to the gas stream for 1hour. Samples 1, 10 and 11 had total conversions of 61, 38 and 54%,respectively, under these conditions.

Example 30

Hectorite from San Bernardino County, California USA, was purifiedutilizing the procedures of Example 1. A 1:1 CaCO₃ :hectorite claycomposite was prepared in accordance with the method of Example 1.Excess chloride ions were removed by dialysis. The composite preparationwas tested for SO₂ reactivity according to the method described inExample 1. After 1 hour of reaction, the composite gave a totalconversion of 85% with 24% of the reaction occurring within the firstminute.

RESULTS OF EXAMPLES

Table 1 lists the values for SO₂ uptake after 1 minute and after 1 hourfor all of the Examples 1 to 29. Unless otherwise specified, the sampleswere ramped at 5° C./min until a temperature of 900° C. was reached. Thetemperature was maintained at 900° C. for 30 minutes prior to admittanceof 5000 ppm of SO₂ into the gas stream for 1 hour. The sample wasmaintained at 900° C. for an additional 30 minutes after admission ofSO₂ had ceased to test the thermal stability of the products.

                                      TABLE 1                                     __________________________________________________________________________    Activity for SO.sub.2 Removal Using Base/Clay Composites                                      Rxn.  % Conversion.sup.a                                      S.#                                                                              Base/Clay                                                                              Ratio                                                                             Temp. °C.                                                                    1 min.                                                                            60 min.                                                                           Comments                                        __________________________________________________________________________    1  CaCO.sub.3 /Na-mont                                                                    1:1 900   19  88                                                  2           3:1 900   14  100                                                 3           0.33:1                                                                            900   36  100                                                 7           1:1 900   13  52                                                  8           0.33:1                                                                            900   15  59  unwashed                                        9.sup.b     1:1 900    6  42  unwashed                                        10          1:1 900    5  43  FeCl.sub.3 doped                                11          1:1 900   13  88  Fe(NO.sub.3).sub.3 doped                        12 Ca(OH).sub.2 /                                                                         1:1 900   .sup.c                                                                             5                                                     Na-mont                                                                    13          3:1 900   .sup.c                                                                             1                                                  14 Ca(FOR)/ 1:1 900   24  60                                                     Na-mont                                                                    15          3:1 900   14  94                                                  16 Ca(ACE)/ 1:1 900   25  52                                                     Na-mont                                                                    17          1.5:1                                                                             900    4  36                                                  18          0.33:1                                                                            900    8  15                                                  19          5:1 900   26  78                                                  20 NaHCO.sub.3 /                                                                          1:2 900   11  33                                                     Na-mont                                                                    21          1:2 500   16  46                                                              1:2 100   active                                                                            active                                              22 CaCO.sub.3 /Na-mont                                                                    1:1 500   .sup.c                                                                            .sup.c                                              23          1:1 700    4  11                                                  24          1:1 800   12  75                                                  25          1:1 800   34  111 pH 20 0.22                                      26          1:1 800   17  64  pH 20 0.064                                     27          1:1 700    7  61  preheat 900° C.                          28          1:1 700   12  38  preheat 900° C.                          29          1:1 700    9  54  preheat 900° C.                          30 CaCO.sub.3 /Na.sup.+-                                                                  1:1 900   24  85                                                     hectorite                                                                  4  CaCO.sub.3 /Bentone                                                                    1:1 900   14  98                                                  5  CaCO.sub.3 /Nontron                                                                    1:1 900   26  78                                                  6  CaCO.sub.3 /                                                                           1:1 900   11  90                                                     Fluorhect                                                                     CaCO.sub.3   900    9  82                                                     precipitated                                                               __________________________________________________________________________     ##STR2##                                                                      .sup.b dried at 100° C.                                                .sup.c no significant uptake.                                            

The present invention has been particularly set forth in terms ofspecific embodiments thereof, it will be understood in view of theinstant disclosure, that numerous variations upon the invention are nowenabling to those skilled in the art, which variations yet reside withinthe scope of the instant teaching. Accordingly, the present invention islimited only by the hereinafter appended claims.

We claim:
 1. A method for preparing a composite material useful forSO_(x) removal from a gas stream which comprises:(a) providing asuspension of a smectite clay in water; (b) providing an alkaline earthmetal salt in the suspension with the clay and with a base so that analkaline earth metal basic compound is formed by a reaction of the saltand the base; and (c) drying the suspension to provide the compositematerial, wherein when the composite material is heated, the SO_(x) isremoved from the base by the basic compound.
 2. The method of claim 1wherein the weight ratio of salt to clay is between about 1:3 and 5:1.3. A method for preparing a composite material capable of removingSO_(x) from a gas stream comprising the steps of:(a) providing asuspension containing a smectite clay in water; (b) dissolving an amountof sodium carbonate in the suspension of the clay; (c) adding a solublealkaline earth metal salt in stoichiometric amount for reaction with thesodium carbonate to form an alkaline earth metal carbonate precipitatein the suspension with the clay; and (d) drying the suspension toprovide the composite material, wherein when the composite material isheated, the SO_(x) is removed from the gas.
 4. A method in accordancewith claim 3, wherein the alkaline earth metal carbonate and clay arerecovered from the water for drying by centrifuging.
 5. A method inaccordance with claim 4 wherein the alkaline earth metal carbonate andclay recovered by centrifuging is washed and air dried.
 6. A method inaccordance with claim 4 wherein the alkaline earth metal carbonate andclay recovered by centrifuging is air dried at a temperature up to about7. A method in accordance with claim 3 wherein the alkaline earth metalcarbonate and clay are mixed with an iron salt selected from the groupconsisting of ferric chloride and ferric nitrate
 8. A method inaccordance with claim 3 wherein the smectite clay is selected from thegroup consisting of montmorillonite, fluorohectorite, bentonite,nontronite, hectorite, saponite, and beidellite.
 9. A method inaccordance with claim 3 wherein the alkaline earth metal is selectedfrom the group consisting of magnesium and calcium.